Linear-beam microwave tubes essentially comprise an electron gun having a cathode providing a cylindrical beam of electrons in an evacuated cylindrical envelope of a microwave structure of the tube. A collector, at one end of the microwave structure, gathers the electrons of the beam exiting the cylindrical envelope.
The electrons exiting the cathode are focused in the form of a linear beam in the evacuated cylindrical envelope by means of a magnetic field. This magnetic field may be created either by permanent magnets, or by windings around the evacuated cylindrical envelope.
FIG. 1 represents a prior art klystron comprising an electron gun 10, a microwave structure 20 comprising resonant cavities C1, C2, C3, C4 and drift tubes in the case of a klystron and, an electron collector 24.
The microwave structure 20 is the element of the tube comprising the evacuated cylindrical envelope with axis Zs, Zs′ where an interaction is performed between a linear electron beam 26 and an electromagnetic wave which may be, either applied to a radiofrequency input Pe 28 of the tube in the case of amplifier tubes, or created in the tube in the case of tubes operating as microwave oscillators. More precisely the electron beam 26 gives up part of its kinetic energy to the electromagnetic wave in the microwave structure. The tube comprises a microwave power output Ps 30.
The electron gun of the tube is often a gun of Pierce type. FIG. 2 shows a simplified cross-sectional view of an electron gun for electron tube of the prior art.
The electron gun of FIG. 2 comprises a spherical-dish-shaped cathode 50 with axis of revolution Zc,Zc′ providing the electrons of a linear beam 52, an electrode 54 serving for the shaping of the beam (also designated by the term wehnelt). The electron beam emitted by the cathode-wehnelt assembly is in general convergent, that is to say that its diameter decreases on moving away from the cathode.
An anode 56 of the tube in front of the cathode-wehnelt assembly accelerates the electrons of the beam exiting the gun along the said axis Zc,Zc′.
The electron gun possesses a symmetry of revolution about the axis Zc,Zc′ which is also the axis of the cathode 50.
The cathode 50 comprises a point O on the axis Zc,Zc′, designated also by centre of the cathode, such that when the cathode rotates about the said centre O by an angle θ with respect to the axis Zc,Zc′ this point O remains on the axis Zc,Zc′.
The beam 52 also possesses a symmetry of revolution about the axis of revolution Zc,Zc′ of the cathode.
When the electron beam has finished converging it attains its minimum diameter, and then diverges under the effect of the electrostatic forces due to the space charge. To keep the beam at the desired diameter it is necessary to use a magnetic field generated by an electromagnet or by permanent magnets situated around the evacuated cylindrical envelope. This magnetic field possesses a symmetry of revolution about the axis Zs,Zs′ of the microwave structure.
The magnetic fields around the beam, along the evacuated envelope, are created by devices called focusers.
FIGS. 3a and 3b show two types of focusers of the electron beam in a microwave tube.
FIG. 3a shows a focuser of a tube comprising a solenoid 60 energized by an electric current producing a magnetic focusing field parallel to the axis Zs,Zs′ of the microwave structure of the tube. An electron gun 62 provides a linear beam 64 in an evacuated circular cylindrical passage 66 of the structure.
FIG. 3b shows another type of focuser comprising a set 70 of n permanent magnets p1, . . . pi, . . . pn of toric shape, coaxial with the axis Zs,Zs′ of the structure creating an alternating magnetic field along the said axis Zs,Zs′.
The circular cylindrical passage 66 of axis Zs,Zs′ for the electron beam in the microwave structures of the tubes, and whose diameter is close to the diameter of the beam, is also the zone of interaction between the microwave structure and the beam.
In theory, the axis Zc,Zc′ of the electron beam emitted by the cathode tied to the gun and the interaction zone axis Zs,Zs′ tied to the microwave structure must coincide. In practice, when the gun is joined to the microwave structure to form the electron tube, the positioning of the gun (and consequently of the cathode) and of the microwave structure is not the desired one, giving rise to a defect of positioning of the gun.
This defect of positioning of the gun can be expressed by means of 5 parameters that can, for simplification, be reduced to three:                3 parameters giving the coordinates of the actual position of the centre of the cathode Or with respect to the theoretical position Ot in a reference frame tied to the axis Zs,Zs′        2 angular parameters giving the inclination of the axis Zc,Zc′ with respect to the axis Zs,Zs′.        
Concerning the defect of positioning of the point Or, it is possible to go from 3 parameters to 2 by considering the plane XY perpendicular to Zs,Zs′ and passing through the point Or. The distance between the point Ot and the point of intersection O1 of the plane XY with the axis Zs,Zs′ corresponds to a distance defect that can be corrected by a translation parallel to the axis Zs,Zs′. In the plane XY the distance Or O1 between the point Or and the axis Zs,Zs′ corresponds to a concentricity defect which involves the 2 coordinates of Or in the plane XY. It is possible without loss of generality to make the axis X pass through the point Or (FIG. 4).
In a system of cylindrical coordinates about an axis Z″ parallel to Zs,Zs′ passing through the point Or, the direction of the axis Zc,Zc′ may be described by 2 angles:                An angle φ giving the bearing in the plane XY        An angle θ giving the elevation (between the axes OrZ″ and OrZc′). It is this angle θ which characterizes the defect of parallelism between the axes Zs,Zs′ and Zc,Zc′.        
When the concentricity defect is zero (Or on the axis Zs,Zs′) the parallelism defect of the gun does not depend on the angle φ when the gun and the line have symmetry of revolution. When the parallelism defect is zero (θ=0) and the concentricity defect is nonzero, the gun positioning defect also does not depend on the angle φ when the gun and the evacuated cylindrical envelope of axis Zs,Zs′ have symmetry of revolution. On the other hand, for nonzero concentricity and parallelism defects, the gun positioning defect depends on the bearing φ: indeed, it is not the same thing to inject a beam in the direction of the axis Zs,Zs′ and at 180° to this direction. The objective being to obtain θ=0, it is indeed this angle that will be taken in order to characterize the parallelism defect of the gun. In conclusion, the gun positioning defect will be characterized by the following 3 quantities:                Distance defect OtO1         Concentricity defect O1Or        Parallelism defect.        
The tubes of the prior art comprise devices for adjusting the position of the gun with respect to the structure so that the centre O of the cathode and the theoretical point Ot on the one hand and the axes Zs,Zs′ and Zc,Zc′ on the other hand are made to coincide.
FIG. 5 shows a partial view of a tube of the prior art comprising a gun equipped with a device for adjusting its position in the tube.
More precisely, FIG. 5 shows a sectional view, in an axial plane P, of an electron gun 80 mounted on the acceleration anode 82 of an electron tube. The acceleration anode 82 is secured to the microwave structure of the tube, on the axis Zs,Zs′, and comprises a hole 83 in the said axis Zs,Zs′ for the passage of the electron beam into the microwave structure of the tube.
The circular cylindrical shaped gun 80 comprises a circular envelope 84 of ceramic material with axis Zc,Zc′ having the cathode-wehnelt assembly 86 held on the axis Zc,Zc′ of the gun by a conically shaped skirt 90 secured, by one 92 of its two edges, to the circular envelope of the gun and, by its other edge 94, to the cathode-wehnelt assembly 86.
The gun of FIG. 5 is surrounded by an adjusting collar 100 coaxial with the axis of the gun ZcZc′ secured to the circular envelope 84 of the gun and comprising tapped holes 102 with adjusting screws 104 for adjusting the position of the cathode-wehnelt assembly 86. For this purpose the adjusting screws 104 are in contact with the surface of another collar 106, coaxial with the axis of the microwave structure Zs,Zs′, secured to the acceleration anode 82 of the tube.
A deformable bellows 108 surrounding the gun 80 ensures the leaktightness of the gun and of the evacuated microwave structure.
The mounting of the gun onto the anode 82 secured to the microwave structure of the tube customarily exhibits a distance defect that one tries to minimize by translating the gun, and therefore the centre O of the cathode, by an action on the adjusting screws 104 so as to bring it closer to the theoretical centre Ot on the axis Zs,Zs′ of the microwave structure (see FIG. 4).
By translating with the adjusting screws 104 the gun 80 so that the points P1 and Ot correspond to the same abscissa on the axis Zs,Zs′, there remains a radial gap between the points O1 and Ot giving rise to the concentricity defect. By adjusting the screws 104 differently, it is possible to make the gun tilt and thus to correct the parallelism defect; on the other hand this arrangement does not make it possible to correct the concentricity defects which require a displacement transverse to the axis Zs,Zs′.
In the linear-beam tubes of the prior art, the defects of concentricity and of parallelism of the axes of the beam and of the interaction zone of the microwave structure result from the fitting and mechanical overhaul operations performed on the gun and tube body sub-assemblies. A few guns are furnished with systems for position adjustment by deformation like those of FIG. 5 with a deformable bellows to maintain vacuum-tightness, but, in general, the tilting of the cathode-wehnelt assembly does not make it possible to correct both the concentricity defect and the parallelism defect, the correction of a concentricity defect possibly producing a parallelism defect, and vice versa.